If the subgroup-variable is numeric, the order of the observations is not checked. The horizontal axis is scaled so that the subgroups are spaced uniformly. Formatted subgroup variable values are used to label the major tick marks.
You can use a character subgroup variable to avoid gaps between groups of points or time values on a control chart. You can also use a character subgroup variable to create a chart in which the order of the points depends only on the order in which the subgroups are arranged in the input data set.
You should verify the order of the observations in the input data set before you use a character subgroup variable in conjunction with the TESTS= option. With the exception of Test 1, the tests for special causes are applicable only if the subgroups are provided in chronological order. See Tests for Special Causes: SHEWHART Procedure for details.
To avoid collision of adjacent tick labels on the horizontal axis, the labels are thinned by default. You can override this default with the TURNHLABELS option or with tick label options in an AXISn statement specified with the HAXIS= option.